OBJECTIVE: : To estimate the possible association between spontaneous early preterm delivery and maternal thyroid dysfunction in early pregnancy.METHODS: : Maternal serum concentrations of thyroid-stimulating hormone (TSH), free thyroxine, antithyroperoxidase, and antithyroglobulin antibodies at 11-13 weeks of gestation were compared in 102 singleton pregnancies, resulting in spontaneous delivery before 34 weeks and 4,318 normal pregnancies delivering after this gestation.RESULTS: : In the preterm delivery group, compared with the normal outcome group, there was no significant difference in antithyroid antibody positivity (16.7% compared with 16.8%). In the antithyroid antibody-negative pregnancies in the preterm delivery group, compared with the normal outcome group, the median free thyroxine multiple of the median was reduced (0.94 compared with 0.99 multiple of the median, P<.001), but the median TSH multiple of the median was not significantly different (0.99 compared with 1.01 multiple of the median, P=.331).CONCLUSION: : In pregnancies resulting in spontaneous early preterm delivery, there is no evidence of increased prevalence of antithyroid antibody positivity or maternal thyroid dysfunction at 11-13 weeks.LEVEL OF EVIDENCE: : II.